Nearly 128,000 people are missing inside a sprawling system of secret prisons run by the Syrian government. Hundreds of thousands of people are believed to have passed through it since the Syrian uprising began in 2011, as the authorities used torture — and the fear of it — to crack down on opposition to President Bashar al-Assad.

Here is what we know about the brutal system that has been critical to Mr. al-Assad’s success in crushing an eight-year revolt.

Documents show that top Syrian officials knew of atrocities.

The Syrian government has denied the existence of systematic abuse but government memos smuggled out of the country show that officials who reported directly to Mr. al-Assad ordered crackdowns on civilians and knew of atrocities. They ordered “harsh treatment” of specific detainees and complained of increasing detainee deaths as corpses piled up and decomposed.

One government memo urged personnel to complete paperwork to protect officials from future prosecution.

The prison system is an integral part of the war effort.

While the Syrian military, backed by Russia and Iran, fought armed rebels for territory, the prison system was the government’s main weapon against the civilian opposition. Imprisonment and torture crushed the civil protest movement and helped drive the opposition into an armed conflict it could not win.

Torture is routine.

Detainees are regularly beaten, hung by their wrists, beaten while crammed inside tires, shocked with electricity and sexually assaulted.

More baroque forms of torture include forcing detainees to act like animals, beat or kill one another, and dousing them with fuel and burning them.

A United Nations panel said conditions amounted to ‘extermination.’

Cells are often so crowded that detainees take turns lying down to sleep.

Food is scarce. Toilets are often absent, diarrhea rampant. Injuries and illnesses, usually left untreated, lead to lingering deaths.

Those taken to military hospitals are also not safe: Patients have been tortured and even killed by the staff.

Far more victims than ISIS.

While ISIS’s brutality has captured world attention, the Syrian prisons have far more victims, accounting for 90 percent of those detained or disappeared during the country’s civil war, according to the Syrian Network for Human Rights. The group has documented the deaths of 14,000 people “under torture” in government prisons.

As Mr. al-Assad nears victory, arrests are increasing.

Syria is fading from world attention as the war winds down. Yet the security forces are ramping up arrests. Detainees have smuggled out warnings that hundreds are being sent to an execution site, and newly released prisoners report that killings are accelerating.

Fear of arrest and torture is one reason that more than five million refugees outside Syria are unlikely to return to the country when the war ends.

Anne Barnard has been the bureau chief in Beirut since 2013. She joined The Times as a Metro reporter in 2007. Previously, she covered the Middle East and the Iraq war for The Boston Globe. @ABarnardNYT